1. Field of the Invention
The present invention relates generally to communication systems, and more particularly, to a method for power control during soft handoffs between second generation (2G) and third generation (3G) (2G3G or 3G2G) Code Division Multiple Access (CDMA) systems.
2. Description of the Related Art
One commonly used type of cellular radiotelephone communication system is referred to as a Code Division Multiple Access (CDMA) system. In a CDMA system, the radio signals share the same frequency spectrum at the same time, in contrast to previous Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) systems. One current CDMA standard, known as the second generation standard or 2G, is designated as TIA/EIA-95-A/B (or IS-95-A/B), and is herein incorporated by reference. More recently, a new third generation (3G) CDMA standard has been proposed and has been designated as IS-2000 (previously IS-95-C) or CDMA2000, and is herein incorporated by reference. As the new 3G systems are installed, cellular systems will contain a mix of both old 2G systems and the new 3G compatible systems.
In a typical CDMA cellular radiotelephone communication system, a mobile station communicates with one or more base stations. In order to track the available signals, the mobile station maintains a list of available base stations, and each station""s relative signal strength. Specifically, each base station in the CDMA system transmits an unmodulated xe2x80x9cpilotxe2x80x9d signal, which is delayed by a base station offset. A mobile station receives the pilot signals and determines which pilot signals are the strongest. A xe2x80x9csearcherxe2x80x9d unit located in the mobile station commonly performs the signal detection and strength measurement functions.
The results from the searcher are reported to the current (i.e. active) base station. The base station then instructs the mobile station to update a list of available base stations maintained by the mobile station. The list is sub-divided into three operative setsxe2x80x94an Active Set, a Candidate Set, and a Neighbor Set. The Active Set contains a list of the base stations with which the mobile station is currently communicating (typically 1-6 base stations). The Candidate Set is a list of base stations which may be moved into the Active Set, and the Neighbor Set is a list of base stations which are being monitored, but less frequently.
As the mobile station moves and its currently active base station signal weakens, the mobile station must access a new base station. Based upon the results of the searcher, and the instructions received back from the base station, the mobile station will update its sets, and communicate with a different base station. In order for communication transmissions to appear seamless to the user of the mobile station, the communication link must be handed off to the next base station. Ideally, this handoff would establish a new link before terminating the first link. This type of handoff is known as a soft handoff (SHO) or xe2x80x9cMake-Before-Break.xe2x80x9d
Techniques for implementing a soft handoff between two different generation base stations are disclosed in U.S patent application Ser. No. 09/314,987, filed May 20, 1999, entitled xe2x80x9cFORWARD LINK INTER-GENERATION SOFT HANDOFFS BETWEEN 2G AND 3G CDMA SYSTEMS.xe2x80x9d The proposed soft handoff solutions, however, rely only on the forward link, and not on the reverse link, due to the incompatibility between the 2G and 3G reverse links. In a CDMA system, a mobile station communicates with a base station via a reverse link, and the base station communicates with the mobile on a forward link. The reverse link is especially important for providing power control information in the CDMA system.
In a CDMA system, each base station attempts to keep the strength of the received signal from all of the mobile stations with which it is communicating at approximately the same strength. This is a central feature in a CDMA system that enables the detection of each mobile station in a multi-user system. Specifically, a base station measures the strength of each arriving mobile signal and then sends power control commands to each mobile station. This power control information is transmitted on the forward link and is known as Reverse Power Control or RPC. The mobile station demodulates and decodes the RPC commands and changes its transmit power accordingly. Note that RPC commands are transmitted on the forward link (base to mobile), but instruct the mobile station to adjust the power on the reverse link (mobile to base).
Additionally, a second power control channel transmitted on the reverse link, the Forward Power Control (FPC) channel, is maintained. The FPC channel assures that the mobile station receives enough signal energy to achieve the desired quality of service (QOS). For the IS-95A/B systems, the FPC is based on the reported frame error rate (FER) of the forward link or on frame erasure indicator bits received from the mobile station. In the IS-2000 system, the mobile station maintains the FPC by measuring the received signal energy and then comparing the energy with a desired QOS. If the received signal energy differs from the desired level, the mobile station sends the power control command on the Reverse Traffic Channel (RTCH) requesting changes to its Forward Traffic Channel (FTCH) transmit power.
The air interface of the new 3G (IS-2000) system uses a new modulation scheme to allow better spectral efficiency, as well as different spreading factors. However, a part of the new 3G system, which operates within the same channel bandwidth as the old 2G system, is required to be compatible with the 2G system at the signaling and call processing level. The reverse link of the 3G system, though, employs coherent demodulation, whereas the reverse link of the 2G system employs non-coherent demodulation. Thus, in the 3G specification, there was no attempt to make these two systems compatible at the physical layer.
Additionally, the forward links of the two systems use different modulation methods (QPSK (3G) vs. BPSK (2G)), which require some modifications within the new 3G system""s demodulator. However, since the IS-2000 terminal (i.e. mobile station) must be able to operate in the IS-95-A/B network, the new 3G terminal is able to switch its mode of operation from one system to the other automatically. In practice, it is impractical to perform a reverse link SHO between 2G and 3G systems because the 3G base station cannot demodulate a 2G reverse link and vice versa (coherent vs. non-coherent, different modulations, etc.).
As a mobile station migrates from a xe2x80x9ccurrentxe2x80x9d generation base station to an xe2x80x9cotherxe2x80x9d generation base station, only the Forward Traffic Channel (FTCH) is in a soft handoff mode, while the Reverse Traffic Channel (RTCH) is maintained with the xe2x80x9ccurrentxe2x80x9d generation base station. This is due to the impracticality of building a mobile station capable of simultaneously processing and modulating signals for an IS-95A/B and an IS-2000 reverse link. For the CDMA FTCH, however, the mobile station is capable of demodulating two different generation signals, since it contains a Rake receiver, containing several demodulating elements.
Since the mobile station only maintains the reverse link with one generation of base stations during an inter-generation soft handoff (ISHO), closed loop power control is only possible with those base stations. As such, during ISHO, the xe2x80x9cotherxe2x80x9d generation base station will not receive the mobile station reverse link until successful termination of the ISHO procedure, thus disrupting its power control processing. In particular, each base station attempts to provide master power control processing for all the mobile stations with which it is communicating. Since the reverse link is not used during an ISHO, the xe2x80x9cotherxe2x80x9d generation base station is unable to hear the mobile station.
During an ISHO, the xe2x80x9cotherxe2x80x9d generation base station, while measuring the energy of the nonexistent RTCH, may request an increase in the power of the channel from the mobile station. Simultaneously, the base station may also increase its transmit power under the assumption that the mobile station has an impaired channel condition (i.e. shadow fading). Assuming that the sudden increase in the power of the mobile station which is in ISHO will change the power balance at the input to the base station receiver, the base station may also request that all other mobile stations in its service area increase their transmit power. Both of these events, the increase of the FTCH power dedicated to the mobile station during ISHO, as well as the increase of transmit power to all other mobile stations, will have a significant negative impact on the service quality to other mobile stations and on the capacity of the system. Thus, there is a need for an improved method to control power during an inter-generation soft-handoff.
The present invention is a method for power control during soft handoffs for a multi-user CDMA system having mixed system types, such as IS-95A/B and IS-2000. The present procedure includes modifying the Forward Power Control (FPC) and Reverse Power Control (RPC) processing, in order to maintain proper power control between a mobile station and two different base stations during a soft handoff.
The mobile station maintains the forward and reverse power control while still receiving forward links from different generation base stations, and while maintaining the reverse link to only the first base station. Prior to the soft handoff, the second base station suspends FPC and RPC processing. Once the soft handoff is complete, the second base station resumes FPC and RPC processing. Furthermore, the mobile station""s initial transmit power level is set to be within a predetermined limit. The present procedure limits the interference to the other users on the forward and reverse channel, while still allowing xe2x80x9cmake-before-breakxe2x80x9d transitions from one generation system to another generation system.